Electric vehicle (EV) wireless power transfer is a technique of transferring power from a power supply unit connected to a power source to an electric load of an EV through a magnetic field. Resonance circuits of various circuit topologies may be used to transfer wireless power between the power supply unit and the EV.
In a conventional resonance circuit example, a structure (hereinafter, referred to as “Series-Series (SS) structure”) includes a primary-side compensation capacitor CP connected in series with a primary coil LP and a secondary-side compensation capacitor CS connected in series with a secondary coil LS to compensate for inductance, as shown in FIG. 1. A wireless power transfer (WPT) system using the above-described SS circuit topology has the simplest capacitor design since the compensation capacitors are connected in series in each of the primary and secondary coil sides. However, the system is inadequate for battery charging since it exhibits current source output characteristics.
For example, as shown in FIGS. 2 and 3, a 3.3 kW conventional WPT system designed at a resonance frequency of 85 kHz exhibits a constant current (IO) characteristic at the corresponding frequency. The system may have a problem in which the phase ϕ of the input impedance Zin is inverted due to the bifurcation phenomenon under light-load (e.g., 0.41 kW).
Also, as shown in FIG. 4, another conventional WPT system may have a structure (hereinafter, referred to as “Series-Parallel (SP) structure”) in which the primary-side compensation capacitor CP is connected in series with the primary coil LP and the secondary-side compensation capacitor CS is connected in parallel with the secondary coil LS. A WPT system using the SP circuit topology exhibits a constant voltage (VO) characteristic, and thus the problem of controlling the output voltage (VO) can be resolved. However, the bifurcation phenomenon in which the shape of the phase curve changes according to the operating frequency, depending on the design, causes the stability of the entire system to deteriorate, causing the control to become difficult, and making the system design very difficult.
For example, as shown in FIGS. 5 and 6, a 3.3 kW conventional WPT system designed at a resonance frequency of 85 kHz exhibits a constant voltage characteristic at the corresponding frequency and is suitable for battery charging. However, similar to the SS circuit topology, the bifurcation phenomenon may occur at a light-load (e.g., 0.41 kW), which is inadequate for light-load operation.
Also, as shown in FIG. 7, yet another conventional WPT system may have a structure (hereinafter, referred to as “Inductance-Capacitance-Inductance-Series (LCL-S) structure”) in which an LC resonance circuit having an inductor Lin connected in series to the primary coil LP and a capacitor CP connected in parallel to the primary coil is connected to the primary coil LP, and a compensation capacitor CS is connected in series to the secondary coil LS. A WPT system using the LCL-S circuit topology exhibits a constant voltage output characteristic and exhibits a bifurcation-resistant characteristic as compared to the SS or SP circuit topology. However, the inductance value of the primary coil LP for satisfying output power and voltage conditions is relatively small. Therefore, the WPT system using the LCL-S circuit topology has a drawback in that the system design is difficult due to a very large primary-side coil current.